Known per se is how to control the gap between the die as a function of one parameter of pellet quality via the machine control means. See EP-A No. 1 0 040 406. The invention herein permits, with simple means, achieving a trouble-free positioning of the press rollers that can be controlled externally, either by entering a set point value or by regulating as a function of operating parameters.
The pellet mill in accordance with the invention is characterized by the fact that the displacement mechanism is provided with a means for regulating the position of the press rollers. Possible with this is to regulate the position of the press rollers not only in the desired direction, but rather also to control the extent of displacement as a function of the magnitude of the deviation from a set point to the parameter to the regulated, or possibly the exact press roller position being strived for.
Shown in EP-A2 0 101 614 is a pellet mill whose adjusting mechanism cooperates with a means for measuring the position of the press roller. This pellet mill includes a flat, horizontal die that is attached to the press frame. The press rollers are disposed on horizontal axles that are attached to a supporting head. The supporting head is mounted in rotation-fast but axially displaceable fashion on a vertical shaft driven by the press motor. Mounted on a radial arm on the supporting head is at least one vertical, triangular measurement flag. Attached to the machine frame is a non-contacting sensor that is activated by the measuring flag travelling past. The travel time of the measurement flag past the sensor is a measure of the axial position of the supporting head on the drive shaft and, therewith, of the gap between the press rollers and the die. This measurement can influence the regulation of the press and/or can itself be controlled by the press regulating means. Because the measurement flag is attached to the supporting head and the sensor is attached to the machine frame, there exists the possibility of undesireable influencing of the sensor by the product instead of by the measurement flag. The travel time of the measurement flag past the sensor depends not only on the axial position of the supporting head on the drive shaft, but also on the rotation speed of this drive shaft, which complicates regulation.
The pellet mill in accordance with the invention avoids this problem in an astonishingly simple manner. It is characterized by the fact that the position measuring means of the position controller is constructed as part of the adjusting mechanism. In this fashion, this position controller is protected against disturbances by the product and by the influence of other press parts.
Construction becomes simple if the adjusting means includes a hydraulic cylinder/piston unit that is provided with a measuring means for the position of the piston relative to the cylinder. Alternatively, however, adjustment of the press rollers for at least one press roller can be accomplished by at least one electronically controlled screw spindle, which then has the same function as the cylinder/piston unit as a positioning member.
Advantageously, the means for regulating position includes a regulating valve with a set point input for the position of the piston, and with a feedback means for the operating valve of the piston position with the regulating valve acting upon the cylinder/piston unit in accordance with the direction of the difference between operating and set point value.
For obtaining and maintaining a precise position, the piston is preferably designed as a double-acting piston. Compared with pistons having mechanical counterforce elements, such as for example springs, this has the advantage of there being present no travel-force dependency. The adjusting mechanism, particularly the set point value input of the regulating valve, can be connected to the machine regulating means.
It is possible to mount the regulating valve on the cylinder of the cylinder/piston unit.
Depending upon the construction of the pellet mill, the cylinder/piston unit can adjust the press roller directly in a predominantly radial direction. In most cases, however, it grips onto an eccentric stud serving to support and adjust the press rollers.
If the piston/cylinder unit is disposed on the press roller carrier within the working space of the mill, the hydraulic and electrical lines are guided, via rotating connections, through the shaft of the roller carrier that is disposed in fixed fashion in operation. In this fashion, these lines are not impaired by the rotation of this roller carrier, which occurs in the case of overloading the mill if the breakaway stud that secures this roller carrier against rotation, breaks. It is possible to control the position of the press rollers externally even after a break, in particular in order to eliminate a blockage in more simple fashion.
This latter advantage is obtained even if the piston/cylinder unit is disposed on the doors, outside the working space of the mill. Accessability to this cylinder/piston unit is improved for maintenance.
It is possible to provide one cylinder/piston unit for each press roller, however, as well as to allow one single unit of this type to act on several press rollers.
In the event of overloading the mill, which is most often signalled by excessive current consumption of the drive motor, it is possible, after interrupting power, for the piston/cylinder unit to remove the press roller from the die and then again be adjusted, in steps, in the direction of the die until the gap between press rollers and die is equal to zero. When the position of the press rollers, stored in the machine regulating means, is reached with gap equal to zero, one knows that the die and the press rollers are again clean and that it is not necessary to open the machine: the mill can again be driven under power.
Precise regulation is strongly improved if the amount of wear of the pairing of die/press roller is entered and stored in the machine regulating means as a function of operating time, and/or of quantity of product produced and serves for correction of the gap between the press rollers and the die. With a cleaner press, it is also possible, when the gap is equal to zero, to establish the positions of the press rollers by raising the pressure in the cylinder/piston unit, and to store this in the machine controlling means. With this, also capable of being controlled in accordance with need, is the stored function: wear=f(time). This construction and method of operation also provide a great advantage relative to protection of personnel, because the doors need not be opened as often, which reduces the danger of accidents.